Automatic volume control with noise suppression



Sept. 5, 1933. J. 5. STARRETT 1,925,825

AUTOMATIC VOLUME CONTROL WITH NOISE SUPPRESSION Filed Nov. 1, 1932 INVENTOR JOHN .5. 57/152677 BY QM ATTORNEY which method consists cutting on the first audio tube when no signal above a certain minimum I to provide Patented Sept. 5, 1933 PATENT OFFICE AUTOMATIC VOLUME CONTROL WITH NOISE snrPsEssIoN John s. 'Starrett, Livingston, N. J.,

Radio Corporation of America, a corporation of Delaware.

assignor to Application November 1, 1932. Serial No. 640,579 8'Claims. (Cl. 250-) My present invention relates to means for automatically suppressing radio receiving set noise as applied to circuits arranged for automatic volume control.

One of the main disadvantages ofthe ordinary schemes of automatic volume control is the fact that when changing the tuning of the receiver, maximum "sensitivity is automatically secured when no station is being received. This results in-maximum amplification of objectionable tube noises and static with consequent annoyance to the userof the receiver.

Hence, it may be stated that it is one of the main objects of my present invention to provide 1 a means for automatically suppressing the response of a radio receiver to tube noise and static when tuning between stations.

Another important object of the present invention consists in providing a method of sup- Y pressing undesired noises in a radio receiver equipped with automatic volume control, when tuning the receiver to a pomt in the tuning rangewhere little or no carrier is being received, in suppressing noise by is received, the rectifled signal voltage being utilized to remove the aforesaid audio tube from cut-off.

A further object of my present invention is an improvement in automatic noise suppression arrangements for automatic volume control circuits, wherein a noise suppressor tube is employed independently of the usual automatic volume control tube, and wherein the noise suppressor tube is rendered operative or inoperative by direct current variations occurring in the output-circuit of the automatic volume control tube, the noise suppressor tube being employed for controlling the'efiiciency of audio frequency amplification.'

' Still other objects of the present invention are to improve generally the simplicity and efliciency of automatic volume control circuits for radio receivers, and to particularly provide an automatic noise suppression arrangement which is not only durable and reliable in operation, but

economically installed in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, asto both its organization and method of operation will best be understood by reference to the following'description taken in connection with the'drawing in which'I have o indicated diagrammatically one circuit orgamza- I appropriate tion whereby my invention maybe carried into effect.

Referring now to the accompanying drawing, wherein the single figure shows an arrangement embodying the invention, the numeral 1 desig- 6 nates a source of modulated signal energy. This source in the present case comprises the network preceding the intermediate frequency amplifier stage in a superheterodyne receiver. Those skilled in the art are well acquainted with the nature of this network, and briefly it may be stated that the network usually comprises one or more stages of tuned radio frequency amplification, a tunable local oscillator, a first detector, or frequency changer, circuit, the resonant circuit 2.being understood to comprise the output of the first detector. The numeralB is to be understood as designating the, tunable circuits of the radio frequency amplifier, local oscillator and first detector stages, and those skilled in the art are aware of the fact that adjustment of the tuning variable condensers results in appropriate tuningzof the receiver to a desired station.

The network 2 is fixedly tuned to the desired intermediate frequency, which may be anywhere between 175 to 465 kilocycles, and the circuit 2 is coupled, as at M1, to the resonant circuit 4 which is tuned to the said desired intermediate frequency. The intermediate frequenc'y circuits are shown adjustable for factory adjustment. The tube 5, a pentode tube of the radio frequency type employing a suppressor grid at cathode potential, has its control grid connected to the high potential side of the input circuit 4, while the cathode of the tube is connected to the low potential side of the input circuit through a path which includes the lead 6, the lead 7, the volume control resistor R (having a magnitude of about 0.5 megohms),

the lead 8, and the lead 9, the lead 9 being connected to ground through a radio frequency bypass condenser 10.

'The anode of the tube 5 is connected to an point of positive potential on the voltage divider P through a path which includes the outputcircuit 4', fixedly tuned to the desiredintermediate frequency, the lead 11, and the lead 12, the lead 11 being connecte d to ground through a radio frequency by-pass condenser 10'. The screen grid electrode of tube 5 'is connected toa point of less positive potential on the divider P than the anode of tube 5 through a path which includes thelead 13, and

the lead 14, the latterbeing connected to ground through a radio frequency by-pass condenser 10".

The second detector tube 15 is of the same type as the tube 5, and has its control grid connectedto the high potential side of the resonant input circuit 16, which is fixedly tuned to the desired intermediate frequency, the circuit 16 being further coupled, as at M2, to the circuit 4'. The cathode of the second detector tube 15 is connected to the low potential side of the input circuit 16 through a path which includes the grid bias resistor 17, the lead 18, the lead 7, the volume control resistor R, the lead 8,. the lead 9, and the lead 19. The lead 18 is connected to a point on the voltage divider P which is at a less positive potential than the points to which the anodes and screen grids of the tubes 5 and 15 are connected, it being noted that the cathodes of both tubes 5 and 15 are established at the same potential.

The screen grid of tube 15 is connected to the same point on the voltage divider P to which the screen grid of tube 5 is connected, and this tential.

is also true with regard to the anode of tube 15. That is, the anodes of tubes 5 and 15 are connected to the same point of positive potential. The anode circuit of the second detector tube iscoupled, as at M3, to the first audio frequency amplifier tube 20, the coupling M3 being an audio frequency transformer. The tube 26 is a pentode tube of the audio frequency type, and also'employs a suppressor grid at cathode po- The control grid of tube 20 is connected to point 21 on the voltage divider P through a path which includes the secondary 22 of the transformer M3, the lead 23,.a resistor R2 having a magnitude or" about 0.5 inegohms. The cathode of tube 20 is connected to the aforesaid point 21 through a path which includes the grid bias resistor 24.

The screen and plate current thru this resistor is sumcient for the development of normal bias on the grid of tube 20. The screen grid of tube 20 is connected, through a lead 25, to a point on the voltage divider P which is of a higher positive potential than. the point to which the screen grids of tubes 5 and 15 are connected. The anode of the audio frequency amplifier tube 20 is connected to the same point on the divider P to which the anodes of tubes 5 and 15 are connected. The anode circuit of tube 20 is arranged for coupling, as at M4, to the input circuit of a push-pull output stage, the latter being connected to areproducer.

The automatic volume control tube 26 is a pentode tube of the same type as the tube 26, and the noise suppressor tube 27 is also of this same type. The anode of tube 26 is connected to the lead 8, while the control grid of the tube is connected to two circuit elements. One of these elements constitutes a radio frequency path for. the modulated signal energy, and this path comprises the lead 28, the lead 29, and the radio frequency by-pass condenser 30 connected to the control grid of second detector tube 15. The control grid bias for this tube is applied thru the resistor 31 connected between the lead 28 and the negative side of the voltage divider P. The cathode of tube 26 is connected, through a lead 32, to a point of higher positive potential than the point to which the control grid of tube 26-is connected, while thescreen grid of thetube 26 is connected to the cathode of. the noise suppressor tube 27 through .a path which includes the self-bias resistor R1.

The control grid of the noise suppressor tube, 27 is connected to the anode of the volume control tube 26, through a lead 8', the volume control resistor R being'connected in shunt across the control grid and cathode of the tube 27, and in shunt across the anode and screen grid of the volume control tube 26. The screen grid of the suppressor tube 27 is connected by a lead 14 to the same point on the divider P to which the screen grid of the second detector tube 15 is connected, andv the anode of the suppressor tube is connected to the control grid circuit of the audio frequency amplifier tube 20 through a path which includes the lead 23', the lead 23, and the secondary winding 22. It should, also, be noted that the anode of the suppressor tube 27 is connected to one side of the resistor R2, and that a by-pass condenser 40 is connected in shunt across the resistor R2 and the lead 23. The current thru R2 serves to bias the audio tube 20 to cut-off where the proper plate current is flowing thru tube 27. g

The operation of the present invention will now be clearly understood from the above description'and the drawing. The automatic volume control tube 26 is connected to the superheterodyne receiver circuit so as to maintain the input to the second detector at a substantially uniform intensity level. This is accomplished by regulating the bias of the-control grids of the intermediate frequency amplifier tube 5 and the second detector tube 15 in accordance with voltage variations across the resistor R. The drawing also shows the anode of the volume con trol tube 26 adapted for connection to the radio frequency amplifier grids so that the gain of the radio frequency amplifier stages may also be controlled at the same time. This is such a well known expedient, that it is believed sulficient to merely make brief reference to this arrangement.

Assume, now, that the circuit 3 has been tuned to a desired signal frequency in the broadcast range, and that for one reason or another the radio frequency input to the receiver varies froma predetermined intensity level,'which level is one giving a desired reproducer output, If the variation is in an increasing sense, then the input to the second detector will increase. Since the input electrodes of the volume control tube 26 are connected to the input circuit of the second detector, the radio frequency input to the volume control tube 26 will therefore similarly increase. The radio frequency input to tube 26 is impressed on the grid circuit of the tube, causing an increase in the D. C. component of the plate current. The greater the increase of the R. F. input to the tube 26, the greater will be the potential drop across the resistor R which is connected in the anode circuit of the tube 26. 1 V t This voltage drop through. the plate resistor R is applied betweenthe cathode and grid of the radio frequency amplifier tubes, the inter mediate frequency amplifier tube, or tubes,'and the second detector .tuber lf the input to the second detector varies in a decreasing sense with respect to the aforementioned predetermined level, then the reverse action will takeplace and the voltage drop-across the resistor R will decrease. Hencajit. will be seen that when the radio frequency input to the second detector increases above the predetermined level, then the gain of,- the radio frequency, intermediate frequency and second detector stages will be de- 'creased, due to increasednegative bias on the gain control electrodes of the stage, and when the input to the second detector decreases, the gain of the aforesaid stages will be increased.

Now, when the tuning device of the receiver is adjusted to a point in the frequency spectrum where little or no carrier is being received, as between two station points, the volume control tube 26 is at cut-01f; no current flows thru' the resistor R, and the voltage drop across R is substantially zero. Under this condition the control grid of the noise suppressor tube 27 is at a potential which allows sufficient plate current to flow through the resistor R2 to cause a voltage drop which adds to the normal bias in the first audio amplifier stage, thus cutting off plate current in the latter. In other words when little or nov signal carrier is being received, the volume control tube 26 is biased to cut-off, with the result that the negative potential on the control grid of tube 27 decreases to a point such that the plate current of this latter tube increases sufficiently to cause a bias to be applied to the grid. of the audio frequency amplifier tube 20, which in addition to the normal bias thereon, results in rendering the audio frequency tube 20 inoperative. This, of course, prevents. the transmission of noise to the succeeding push pull output stage.

As soon as the signal carrier energy to the second detector. increases, due for example to a station being turned in, the voltage across the resistor R increases, as explained before, and this causes the suppressor tube to be biased to cut-off, because the negative bias on the control grid of tube 27 increases. The plate current 'of the suppressor tube 27 decreases, and removes the additional audio frequency amplifier cut-off bias. Thus the audio frequency amplifier tube is allowed to amplify in normal fashion, and transmit the signal to the succeeding push-pull output stage. Therefore, it will be seen that the noise suppression function is controlled from the output circuit of the volume control tube, and that the noise suppression function comprises rendering the audio frequency transmission of the signal highly inefficient. It is desirable that the automatic volume control action of the receiver be essentialy flat from perhaps 2 microvolts to 200,000 or more in order that the weaker stations be amplified by the audio tube. To secure the most sensitive action the automatic volume control, the noise suppressor and audio tubes should have sharp cut-off characteristics. v

While I have indicated and described one system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limitedto the particular organization shown and described, but that many modifications may be made without departing from the scope of my inventionyas set forth in the appended claims.

What I claim is:

1. In combination with a high frequency amplifier stage including an electron discharge tube having resonant input and output circuits, a detector circuit including a tube having a resonant input'circuit coupled to the resonant output circuit of the first tube, a low frequency amplifier including a tube having its input circuit coupled to the output circuit of said detector tube, an automatic volume control tube including a cathode and at least two cold electrodes, a radio frequency path between one of 2L Incombination between one other cold electrode and suppressor, tube including a cathode said cold electrodes and the resonant input cir cuit .of said detector, means for applying a bias to said one cold electrode such that the volume control tube is normally maintained at cut-off, a direct current connection between the other cold electrode and a gain control, electrode of the high frequency amplifier tube, a noise suppressor tube including a cathode and at least two cold electrodes, a direct current connection between one of the cold electrodes of the noise suppressor tube andv said other cold electrode of the volume control tube, and a direct current connection between the other cold electrode of the noise suppressor tube and an electrode of said low frequency amplifier tube.

with a high frequency aman electron discharge tube and output circuits, a detector circuit including a tube having a resonant input circuit coupled to the resonant output circuit of the first tube, a low frequency amplifier including'a tube having its input circuit coupled to the output circuit of said detector tube, an automatic volume control tube including a cathode and at least two cold electrodes, a radio frequency path between one of said cold electrodes and the resonant input circuit of said detector, means for applying a bias to said one cold electrode such that the volume control tube is normally maintained at cut-off,

plifier stage including having resonant input 'a direct current connection between the other cold electrodeand a gain control electrode of the high frequency amplifier tube, a noise suppressor'tube including a cathode and at least two cold electrodes, a direct current connection of the cold electrodes of the noise suppressor tube and'said other cold electrode of the volume control tube, and a direct currentconnection between the other cold electrode of the noise suppressor tube and an electrode of said low frequency amplifier tube, means for applying a bias to said low frequency amplifier tube electrode such that the latter is rendered inoperative to amplify when said volume control tube is cut-off. V v

3. In combination with a high frequency amplifier stage including an electron discharge tube having resonant input and'o'utput circuits, a detector circuit including a tube having a resonant input circuit coupled to the resonant output circuit of the first tube, a low frequency amplifier including a tube having its input circuit coupled to the output-circuit of said detector tube, an automatic volume control tube including a cathode and at least two cold electrodes, a radio frequency'path between one of said cold'electrodes and the resonant input circuit of said detector, means for applying a bias to said one cold electrode such that the volume control tube is normally maintained at cut-off, a direct current connection between the a gain control electrode amplifier tube, a noise I v and at least two cold'electrodes, a direct current connection between one of the cold electrodes of the noise suppressor tube and said other cold electrode of the volume control tube, and a direct current connection between the other cold" electrode of the noise suppressor tube and an input electrode of said low frequency amplifier tube.

4. In combination with a high frequency amplifier stage including an electron discharge tube of the high frequency having resonant input and output circuits, a detector circuit including a tube having a resonant input circuit coupled to the resonant output circuit of the first tube, a low frequency amplifier including a tube having its input circuit coupled, to the output circuit of said detector tube, an automatic volume control tube including a cathode and at least two cold electrodes, a radio frequency path between one of said cold electrodes and theresonant input circuit of said detector, means forapplying -a bias to said one cold electrode suchthat the volume control tube is normally maintained at cut-off, a direct current connection between the other cold electrode and a gain control electrode of the high frequency amplifiertube, a noise suppressor tube including a cathode and at least two cold electrodes, a direct current connection between one ofthe cold electrodes of the noise suppressor tube and said other cold electrode of the volume control tube, and a direct current connection between the other-cold electrode of the noise suppressor tube and a grid electrode of said low frequency amplifier tube.

5. In combination with a high frequency amplifier stage including an electron discharge tube having resonant input and output circuits, a detector circuit including a-tube havinga resonant input circuit coupled to the resonant output circuit of the first tube, a low frequency amplifier including a tube having its input circuit coupled to the output circuit of said detector tube, an automatic volume control tube including a cathode and at least two cold electrodes, a radio frequency path between one of said cold electrodes and the resonant input circuit of said detector, means for applying a bias to said one cold'electrode such that the volume control tube is normally maintained at cut-off, a direct current connection between the other cold electrode and a gain control electrode of the high frequency amplifier tube, a noise suppressor tube including a cathode and at least two cold electrodes, a direct current connection between one of the cold electrodes of the noise suppressor tube and said other cold-electrode of the volume control -tube, and adirect current connection between the other cold electrode of the noise suppressortube and an electrode of said low frequency amplifier tube, said volume control tube additionally including a third cold electrode,

and a direct current-connectionbetween the oath-- ode of the noise suppressor tube and said third cold electrode of the volumecontrol tube.

6. In combination with-a high frequency amplifier stage including an electron discharge tube havingresonant input and output circuits,

a detector circuit including a tube having .a restector tube, an automatic volume control tube including a cathode and at least two cold electrodes, a radio frequency path between one of i said cold electrodes and the resonant input cirthe volume control tube, and a direct current connection between the other cold electrode of the noise suppressor tube and an electrode of said low frequency amplifier tube and an additional direct current connection between said other cold electrode of the volume control tube and an input electrode of said detector tube.

"7. 'In combination with a superheterodyne receiver of the type including means for tuning the receiver through the broadcast range, an intermediate frequency amplifier, a second detector and at least one stage of audio frequency amplification, an automatic volume control tube including means for normally maintaining the volume control tube biased to cut-off in the absence of received signals, a signal path between the .input of the second detector and the grid of the volume control tube, a direct current connection between the anode of the volume control tube and the grid electrodes of the inter- .mediate frequency amplifier and the second detector, a noise suppressor tube having its input electrodes connected to the output circuit of said volumecontrol tube, and its anode connected to the grid circuit of the audio frequency amplifier, and means in said last named connection for biasing the audio frequency amplifier to cut-off in the absence of signal input to the volume control tube.

8. In combination with a superheterodyne re- ,grid of the volume control tube, a direct current connection between the anodeof the volume control tube and the grid electrodes of the intermediate frequency am'plifierfland the second detector, a noise suppressor tube having its input electrodes connected to the output circuit of said volume control tube, and its anode connected tothe grid circuit of the audio frequency amplifier, and means in said last named connection for biasing the audio frequency ampliifier to cut-off in the absence of signal input to the volume control tube and a single potential source connected to the electrodes of all of said circuits for supplying appropriate potentials thereto.

JOHN S. 'STARRETT. 

